"Display Name" default/always force/only allow First/Last

I have SCOURED the internet and this forum and I have not found one solution that has worked to allow such a basic functionality that should be in the WordPress admin in the first place. I just want the default display name for my users to be First Last, instead of username – preferably forcing that to be the only option.

PLEASE do not give me any links on this forum unless it’s absolutely positively working and recent, I have looked at EVERY ONE and all the solutions are outdated or refer to files that don’t even exist/have anything in them in the current WP version.